What is "tonalite"

n. 1 (context geology English) An igneous, plutonic rock composed mainly of plagioclase 2 (context geology dated English) quartz diorite.

Tonalite is an igneous, plutonic ( intrusive) rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.

In older references tonalite is sometimes used as a synonym for quartz diorite. However the current IUGS classification defines tonalite as having greater than 20% quartz, while quartz diorite varies its quartz content from 5 to 20%.

The name is derived from the type locality of tonalites, adjacent to the Tonale Line, a major structural lineament and mountain pass, Tonale Pass, in the Italian and Austrian Alps.

Trondhjemite is an orthoclase-deficient variety of tonalite with minor biotite as the only mafic mineral, named after Norway's third largest city, Trondheim.